Metabolism of uniconazole-P in water-sediment systems under illumination

Degradation of the strobilurin fungicide mandestrobin in illuminated water-sediment systems

The degradation behavior of mandestrobin (1) was investigated in aerobic aquatic water-sediment systems exposed to continuous artificial sunlight (λ>290 nm). [14C]mandestrobin uniformly labeled at the phenoxy or benzyl ring was individually applied to the overlying water of the system at a rate equivalent to 262.5 g a.i./ha. The transformation of 1 was mainly proceeded via photoinduced bond cleavage at the benzyl phenyl ether and the subsequent rearrangement reaction. Interestingly, some of the photodegradates and microbial metabolites of 1 observed in the aquatic photodegradation and water-sediment (dark) studies, respectively, were never detected. Furthermore, the observed photoproducts were less formed and were steadily degraded or metabolized to carbon dioxide or strongly adsorbed to bottom sediment. The fate of 1 and its degradates in illuminated water-sediment systems was considered to reflect realistic conditions more precisely, as it accounts for various effects attributed to sunlight.

Dissipation of Herbicide Methiozolin and Its Metabolites in Aerobic Sediment-Water Systems

Methiozolin is a novel herbicide for controlling annual bluegrass. After applying ¹⁴C labelled methiozolin in two sediment (clay loam and sand)-water systems under aerobic conditions, its distribution, half-life, and metabolites within 300 days were investigated. The mass balance ranged within 92.0%-104.4% of applied radioactivity (AR). Radioactivity in the water declined sharply from 94.4% to 0.5% AR, while in the sediment it increased to 83.9% AR at 14 days before declining to 9.1% AR. The volatiles were minimal (< 0.5% AR), and the evolved labelled CO₂ accounted for up to ~ 33.4% AR. From Radio-HPLC analysis, labelled methiozolin in water decreased from 108.9% to 0% AR, while a maximum of 15.1% AR remained in the sediment at the end. Eight metabolites were detected, all at minor levels and accounting for < 5.5% AR. The half-life of labelled methiozolin in the total sediment-water systems were 50.7 and 38.7 days for clay loam and sand, respectively.

Theoretical and organic chemical approaches to environmental behavior and metabolism of pesticides

Investigation of the dissipation and transformation of pesticide through laboratory experiments, conducted in accordance with standard and newly developed designs, gives us valuable information to understand their environmental behavior. We have also been investigating the mechanisms of partition and transformation reactions of pesticides, not only through kinetic analyses, but also through theoretical approaches based on their molecular properties estimated using various spectroscopies and molecular orbital calculations. Furthermore, synthetic iron porphyrin with a peroxide was shown to be a good model to simulate the P450-catalyzed oxidation in the metabolism of pesticides. Through these investigations, the knowledge of surface water, soil, sediment, and plants, such as their properties and constituents, was found indispensable to a deep understanding of the mechanism in the hydrolysis, photolysis, and metabolism of pesticides.

Direct photolysis mechanism of pesticides in water

Photodegradation is one of the most important abiotic transformations for pesticides in the aquatic environment, and the high energy of sunlight causes characteristic reactions such as bond scission, cyclization, and rearrangement, which are scarcely observed in hydrolysis and microbial degradation. This review deals with direct photolysis via excitation of a pesticide by absorbing natural or artificial sunlight in order to know its basic photochemistry, and indirect photolysis meaning either sensitization by dissolved organic matters or oxidation by reactive oxygen species is basically excluded. Several experimental approaches including spectroscopic techniques together with theoretical calculations are first discussed from the viewpoint of the reaction mechanisms in direct photolysis. Then, the typical photoreactions of pesticides are summarized by chemical classes and/or functional groups and discussed as far as possible in relation to their mechanisms.

Pesticide behavior in modified water-sediment systems

The standardized laboratory water-sediment study in darkness is utilized as primary information on pesticide behavior to assess its ecotoxicological impacts in the edge-of-field water bodies. The half-lives of pesticide in water and sediment are key parameters to predict its environmental concentration, and its metabolic profiles help to avoid overlooking unexpected toxicological impacts from metabolites. However, no consideration of environmental factors such as sunlight and aquatic macrophytes is included, and this may lead to a conservative assessment. We review the experimental factors in the existing standardized design and then the effects of illumination and aquatic macrophytes introduced to the water-sediment system. The effects of temperature and the water-sediment ratio should be investigated in more detail and the pesticide behavior is possibly modified by illumination via photodegradation and/or metabolism in phototrophic microorganisms. Aquatic macrophytes play a major role as an additional sorption site and in further pesticide metabolism.

Degradation of flumioxazin in illuminated water-sediment systems

The aerobic aquatic metabolism of flumioxazin was studied in two water-sediment systems under illumination and in darkness to investigate its degradation profiles. (14)C-Flumioxazin separately labeled at the 1- and 2-positions of the tetrahydrophthalimide moiety or uniformly labeled at the phenyl ring was applied to a overlying water at a rate equivalent to 600 g ai/ha by assuming uniform distribution in the water layer to a depth of 100 cm. Flumioxazin was rapidly degraded at 20 °C in the overlying waters irrespective of irradiation with half-lives of 0.1-0.4 day. Both various modes of liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR) spectroscopy analyses showed four major degradates under irradiation. Two of them were formed via successive hydrolysis of the cyclic imide ring, and the others were 2-arizidinone derivatives via photoinduced rearrangement. The presence of sediment under illumination greatly reduced the formation of these degradates and accelerated their degradation. The partitions of flumioxazin and its degradates to the bottom sediment not only reduced their fractions in the water layer subjected to hydrolysis and photolysis but also enhanced their microbial degradation in the sediment. The illuminated water-sediment systems were considered to more adequately represent the behavior of flumioxazin and its degradates in the environment than the corresponding studies of aqueous photolysis and water-sediment in darkness.